1. Field of the Invention
The present invention relates to a technology for calculating the amount of misregistration.
2. Description of the Related Art
Image forming apparatuses such as color copiers and printers include tandem image forming apparatuses that are provided with an image forming portion for each color and superimposes toner images of the respective colors to form a color image. With such image forming apparatuses, components such as photosensitive drums and laser scanners become deformed due to the change in temperature of the image forming portions when performing multiple transfers of the different colored toner images, resulting in color misregistered images being formed in which the image formation position of each color is slightly displaced.
In view of this, the following processing is performed in order to correct misregistration of the different colored toner images. First, a patch is formed in each image forming portion, and the amount of misregistration is detected by reading this patch with a sensor. Color registration adjustment for adjusting the image formation timing of each color is then performed based on the detected misregistration amount to thereby prevent formation of color misregistered images.
In a medium-speed or fast-speed color printer of which both image quality and image productivity are required, a considerable amount of heat is needed in order to quickly fix toner images to printing paper. Particularly when the device is powered on from a completely cold state, such as for the first time in the morning, the temperature of the image forming portions (laser scanners, drums, developing units, etc.) inside the device rises rapidly from environmental temperature and approaches the equilibrium temperature of the operating state of the device, while the controller is starting up, device adjustments are being carried out and the fixing unit is warming up. Conventionally, the printer forms the patches and detects the amount of misregistration in this state, and thereafter enters a print ready (standby) state. Although the use of patches to detect the misregistration amount is highly accurate and image quality is kept constant, time is required to form and read the patches, and thus detection cannot be implemented frequently since the user is unable to print during that time and user convenience suffers. Since the temperature of the image forming portions is substantially in equilibrium and changes moderately after the device has warmed up, it is desirable to form the patches at predetermined intervals (every predetermined number of printed sheets or predetermined time period) and detect the amount of misregistration while balancing image quality with user convenience.
With low-speed printers that are mainly for personal use, prediction control that involves storing the relationship between the change in temperature of the device and the amount of misregistration for each color and calculating the adjustment amount by predicting the amount of misregistration according to the change in temperature is mainly used. Although the amount of misregistration can be updated without the user being unable to print, this method is slightly less accurate than the abovementioned detection of misregistration amount performed by forming patches. In Japanese Patent Laid-Open No. 2010-217544, a technology is proposed in which a table indicating amounts of registration adjustment relative to changes in device temperature is stored. Then, when the temperature change is at or below a predetermined value, prediction adjustment based on the adjustment table is performed, and when the temperature change exceeds the predetermined value, the amount of misregistration is measured using patches and the adjustment table is updated.
However, there are the following problems with the above conventional technologies. For example, fixing units capable of warming up on demand with a heating method using induction heating or the like have been developed in recent years in consideration of user convenience. With such image forming apparatuses, high-speed startup is possible even when the device is powered on from a completely cold state, and controller startup and device adjustment operations are also completed and a print ready (standby) state is achieved in approximately 30 seconds. Thus, patches are formed and the amount of misregistration is detected during the period in which the temperature around the image forming portions is rising rapidly from environmental temperature. Since the temperature of the image forming portions continues to rise rapidly for several minutes immediately after startup, image quality deteriorates when printing is performed during this time due to the change in the amount of misregistration that occurs as a result of temperature change after registration adjustment.
On the other hand, prediction of the amount of misregistration can be sufficiently expected to improve during the period in which the temperature is rising rapidly at the beginning of startup. However, in a state where the device has warmed up sufficiently and has stabilized, prediction control results in adjustment accuracy that is inferior to conventional devices that form patches. Also, in the case where the above conventional technologies are applied to a printer with high-speed startup capability, formation of patches and measurement of the amount of misregistration will be frequently performed, since there is a large change in temperature inside the device immediately after startup. Accordingly, even if high-speed startup is performed, image formation will be delayed and user convenience will be adversely affected.